Typical examples of previously known micro-dispensing pipets of the type employing snap-on precision dispensing tubes are illustrated respectively in U.S. Pat. Nos. 4,023,716 and 4,165,646 to J. J. Shapiro. These dispensing pipets employ plunger elements which are adjustable in the dispensing tubes to provide a definite preset delivery volume. Calibration rods are employed to set the plunger elements in a position spaced from the open end of the tip in accordance with the length of a selected calibration rod, which corresponds to a desired delivery volume. In U.S. Pat. No. 4,023,716, the dispensing tube may be transparent and marked with scale lines to indicate volumetric settings. In U.S. Pat. No. 4,165,646, a helical volume scale is provided on the pipet barrel, readable through a notch or window in a guard sleeve relative to which the barrel is adjustable. In each case, the numerical volumetric data obtainable is relatively imprecise and is somewhat difficult to observe. Hence, there is a definite need to improve the precision and accessibility of the observed numerical volumetric data.
In U.S. Pat. No. 4,362,063 to M. d'Autry there is shown a dispensing pipet wherein a plurality of volume-indicator drums are employed in a decade configuration drivingly controlled by a prismatic control rod which is used to set the position of a stop bushing limiting the travel of the pipet dispensing piston. The decade action is provided by the use of external pinion sprocket elements which make the decade drum assembly quite bulky. There is a definite need to reduce the bulk of the apparatus by reducing the bulk of the decade drum configuration, as well as to simplify the mechanical assembly associated with the adjustable stop bushing.